The present invention relates to a method of coiling an optical fiber forming the ring of a gyroscope. A known type of gyroscope is formed from a ring interferometer, also called SAGNAC interferometer.
Such an interferometer comprises mainly a light energy source formed generally by a laser; an optical device formed either by a number of mirrors or by an optical fiber wound on itself, this device forming a wave guide; a device for separating and mixing the light and a device for detecting and processing the detected signal.
It is known that in these interferometers there exist two waves coming from the separator device and travelling in opposite directions over the same optical path.
A fundamental property of ring interferometers is the reciprocity which may be expressed as follows: any disturbance of the optical path affects the two waves in the same way despite the fact that these two waves are subjected to it neither exactly at the same time nor in the same direction.
There exist however two types of disturbances which affect the reciprocity.
These are, on the one hand, disturbances which vary in time, in a lapse of time comparable to the time taken by the waves for propagating along the optical path of the interferometer; and on the other hand, so called "non reciprocal" disturbances, that is to say disturbances which do not have the same effect on the waves depending on whether they propagate in one direction or in the other along the optical path. These are physical effects which destroy the symmetry of the medium in which the waves propagate.
Two known effects present this latter type of disturbance:
the Faraday effect, or colinear magneto-optical effect, by which a magnetic field creates a preferential orientation of the spin of the electrons of an optical material;
and the Sagnac effect, or relativistic inertial effect, in which the rotation of the interferometer with respect to a Gallilean reference destroys the symmetry of the propagation time. This effect is used for forming gyroscopes more particularly.
The invention is situated in this field of application.
In the absence of "non reciprocal" disturbances, the phase difference (which will be called hereafter .DELTA..phi.) between the two waves which are recombined in the separation and mixing device after travelling over the optical path is zero. In the opposite case, that is to say when the system rotates in inertial space, the phase difference obeys the relationship: ##EQU1## in which relationship f is the frequency of the optical wave and C the speed of light in a vacuum.
The phase shift .DELTA..phi. depends on the scalar product between the apparent surface vector S of the fiber coil used for forming the ring and the rotation vector .OMEGA.. The system is therefore sensitive to the flow of the rotation vector through the coil.
The apparent surface vector S is defined by the relationship: EQU S=.intg.1/2r(M).LAMBDA.dl (2)
in which relationship the sign .intg. represents the integral on the closed contour defined by the path followed by the line along the fiber, M any point on this closed contour and dl is the progression difference vector.
The direction vector S is parallel to the axis of symmetry of the coil. The system detects then the rotations about this axis and is insensitive to rotations about orthogonal axes. For numerous navigational applications, it is of basic importance that the axis be very stable. This stability, called "laying", may be affected by movements of the different turns of the fiber coil.
The problem is simple to solve in the case of a coil with a single layer. It is possible to form in a tube supporting the coil a helical groove forming a screw thread, of a period greater than the diameter of the fiber, and coiling the fiber in the helical groove thus formed. A problem arises for the second layer. It may be wound back along the support tube in the same direction of rotation, but then the winding pitch is reversed. At each turn, the fiber will have to leave the groove defined by the preceding layer, pass over a turn and come back into the following groove. Such an arrangement is described for example in U.S. Pat. No. 3,102,953, more particularly in FIG. 2. This passage point is uncertain and may vary in particular as a function of the temperature: expansion phenomena. That modifies the orientation of the turn and so affects the laying stability, creating in particular hysteresis if the fiber does not come back into position during a heat cycle. If the direction of rotation in the coiling is now reversed, it is then possible to wind the second layer in the groove defined by the first one, but the Sagnac effect due to the rotation will be cancelled out, just as the self inductance of an electromagnetic coil is cancelled out.
The aim of the present invention is to provide a coiling process for winding the optical fiber forming the ring of a gyrometer which improves the laying stability thereof.